1. Field of the Invention
The present invention relates to a surge absorber for protecting electrical circuits from a surge voltage. The surge absorber comprises an insulating body having a flat surface on which electrically conductive elements are formed leaving minute discharge gaps therebetween.
2. Description of the Prior Art
FIG. 7 shows one of the typical conventional surge absorbers. In FIG. 7, an electrically conductive thin film 41 is formed on the entire external circular peripheral surface of a cylindrical insulating body 40. The thin film 41 is made of carbon, metal oxides, etc. Terminal caps 42, 42 are fitted to the opposite ends of the insulating body 40. The electrically conductive thin film 41 is cut around the surface of the insulating body 40 by a laser cutter or a diamond cutter, etc. so that the thin film 41 is separated into a plurality of ring-shaped segments and micro grooves 43 are formed therebetween. The entire structure of the insulating body 40, the thin film and the terminal caps 42, 42 are sealed in a glass tube 44 which is filled with an inert gas or nitrogen gas. The Japanese published patent application 63-67918 (published 11-14-88) discloses a surge absorber of this type.
FIG. 7 also shows a thermostatic fuse 45 enclosed in the glass tube 44 for breaking the circuit when the environmental temperature reaches a predetermined level. A thermostatic fuse used for a surge absorber is disclosed in the Japanese laid-open patent application 63-99725 (laid-open 5-2-88).
However, when the thin film as described above is cut by laser cutter etc., the width of the formed micro grooves tends to fluctuate as time lapses and the edges of the micro grooves tend to be formed rough. On the other hand, since the discharge breakdowns occur across the micro grooves, the discharge starting voltage is substantially affected by the widths of the micro grooves and the condition of the edges of the micro grooves. The fluctuation of the discharge starting voltage is translated into a degraded quality of the products. Therefore, the groove cutting process requires an extreme attention and accuracy.
Since the groove cutting must be performed to each piece of the products individually, the products are not quite suitable for mass-production and the production is costly.
Referring back to FIG. 7, the conventional surge absorber sealed in a glass tube 44 with a thermostatic fuse 45 has a potential problem in that a splash of fused metal of the fuse 45 reaches the micro grooves 43, thereby causing the surge absorber to be short-circuited. One method of keeping the metal splash from the micro grooves 43 is to place a shielding plate 46 between the fuse 45 and the surge absorbing element. The reliability of this method, however, has not sufficiently been proved.
U.S. Pat. No. 4,727,350 (2-23-88) discloses a surge absorber in which micro grooves are formed on a cylindrical insulating body in a combination of a spiral groove and a linear groove which intersects the spiral groove.
Japanese laid open patent application 62-278781 (laid-open 12-3-87) discloses a surge absorber in which a plurality of electrically conductive film elements are formed on a surface of an insulating body, one of the elements being a grounding terminal, and a plurality of micro discharging gaps are provided between the non-grounded conductive film elements and the grounded element.